Review of L-tyrosine confirming its safe human use as an adjuvant

Although there is a long history of exposure to allergy vaccines containing L-tyrosine, there has been no central publication reviewing its adjuvant properties in animal and human studies together with an assessment of its safe use. This paper summarizes a range of investigational data (unpublished) available to the authors as well as published literature reports. An array of in vitro and in vivo studies showed that L-tyrosine has ideal adjuvant properties, comprising a high adsorptive power for proteins, enhancement of IgG antibody induction with no stimulatory effect on IgE antibody level and action as a short-term depot adjuvant, delaying the bioavailability of allergenic materials rather than directly influencing immunocompetent cells. A series of preclinical safety investigations comprised single-dose parenteral studies in the mouse and rat, repeat-dose parenteral toxicity studies over 28 days in the rat and dog (up to 25 mg kg(-1) day(-1)) plus genotoxicity and local tolerance studies. No signs of toxicity or genotoxicity were seen; repeat-dose toxicity studies showed expected white cell and spleen weight immunostimulatory effects; local-dose site reactions were also seen and were confirmed in local tolerance studies. Findings from a range of clinical studies using allergy vaccines containing L-tyrosine reflected the lack of toxicity seen in animal work and showed evidence of enhanced immunostimulatory activity. Local injection site reactions (a common response to any form of clinical vaccination) in these studies were likely to be due to the presence of L-tyrosine per se. The lack of findings of toxicological concern found during this review supports the hypothesis that L-tyrosine is a safe adjuvant for human use.